1. Field of the Invention
The invention relates to a process and an apparatus for the thermal treatment of pulverulent substances.
2. Description of Related Art
Known heat treatment methods for pulverulent substances take place in batch processes and therefore require long treatment times and are difficult to automate. Moreover, a disadvantage with these known furnace heat treatment processes, in which the material being heat treated is introduced into the furnace in dishes, pans or other containers, is the formation of agglomerations, coarse lumps and caking of the pulverulent substances as well as the occurrence of inhomogeneities that are caused by a non-uniform heat treatment due to temperature gradients in the furnace chamber, as well as by variable gas accessibility depending on the position in the furnace chamber.
Known revolving tubular furnaces have considerable disadvantages in terms of energy consumption due to the large dead volume. Furthermore the material being heat treated is contaminated by parts of the tubing and adhering catalyst residues.
According to EP 0 591 881 1 (DuPont) it is known to produce palladium and palladium oxide powders with particle sizes of around 1 micron by decomposing a Pd-containing aerosol in a heated wall reactor.
This process cannot be used for supported catalysts containing noble metals.
DE 19545455 (Degussa) describes a process for the production of noble metal powders by heat treatment of an aerosol in a flame reactor.
A disadvantage of the flame-heated reactor system is an irregular temperature/time profile as well as the necessity to feed in fuel gases and air, or oxygen, to produce the flame. As a result, the system is disadvantageously complex.
EP 0 537 502 (Degussa) describes a post-treatment process for noble metal powders in a heated wall reactor, in which the noble metal powder is nebulised in a carrier gas.
On account of the high density the powder mist is inhomogeneous however, with the result that particle-particle contact and thus agglomerations occur during the heat treatment.
DD 286523 (Leunawerke) describes a process for the production of a supported silver catalyst by heat treatment of an inert carrier material that has been impregnated with a silver salt solution.
The decomposition of the silver salt on the carrier material is however incomplete, with the result that an additional heating step is necessary in order to activate the catalyst.
JP 63319053 (Tanaka) describes a plasma process for the heat treatment of electrocatalysts for fuel cells, which is carried out in a quartz tube formed in the shape of a coil.
Although this process leads to alloyed noble metal catalysts, it is very complicated and expensive.
RO 54548 describes a continuous heat treatment process in which the catalyst material is dispersed in a carrier gas and is then treated in an electric arc.
According to the prior art, such heat treatments of pulverulent substances are carried out for example in revolving tubular furnaces or stationary furnaces.
The disadvantage of these thermal treatments (calcination) is that, on account of the thermal stress of the material, the metal crystallites applied to the carrier become mobile and sinter.
A thermal treatment of pulverulent catalysts at high temperatures without such sintering effects with simultaneous alloy formation, fixing of the metal on the support, and decomposition of metal oxides or the removal of surface functionalities is not possible according to the prior art.
According to U.S. Pat. No. 4,624,937 or U.S. Pat. No. 4,696,772 it is known to remove surface functionalities by treating a carbon-based carrier material by oxidation with HNO3 and subsequent thermal treatment.
A disadvantage of this process is that it includes a further complicated step involving the production of metal-containing, pulverulent catalysts.
There is therefore a need to develop a process for the thermal treatment of pulverulent substances that does not have these disadvantages.